microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 1 copyright ? 2004 rev. 1.1, 2006-05-15 www. microsemi . com LX1911 1.5a, 1.1mh z pwm step-down regulator p roduction d ata s heet tm ? description the LX1911 operates as a current mode pwm buck regulator that switches to pfm mode with light loads. the entire regulator function is implemented with minimal external components. the LX1911 responds quickly to dynamic load changes using a high bandwidth error amplifier and internal compensation. tight output voltage regulation is maintained with the compensated 800mv, 2% reference (line and temp regulation). with two external resistors the output voltage is easily programmed, from 800mv to 90% of v in . the regulator is capable of providing an output load current of 1.5a and has no minimum load current requirement for stable operation. current limit is cycle-by-cycle to protect the switch. power conversion efficiency is maximized with a low regulator iq and pfm mode of operation. the LX1911 operational range covers 4.0v to 6.0v, features include power on delay; soft start to limit inrush currents; and thermal shutdown during fault conditions. the 6-pin package is a small, rohs compliant / pb-free, mo-229 with excellent power dissipation capacity. important: for the most current data, consult microsemi ?s website: http://www.microsemi.com key features ? internal reference 800mv 2% accuracy (line and temp.) ? 4.0v to 6.0v input range ? internal soft start ? adj. output from 0.8v to v in ? output current (i >1.5a) ? quiescent current < 550 a, typical @ 23c ? 1.1mhz pwm frequency ? micro lead-frame, thin mo- 229, 6-pin package applicati ons/benefits ? portable microprocessor core voltage supplies ? 5v to 3v product highlight vin power vin analog gnd n.c. sw fb 1 3 6 4 5 2 4.7f 4.0 to 6v c 10 3.3h 25k 50k 20f ground range tie-in 1.2v @ 1.5a LX1911 c = 680p efficiency vs i out (v in =5v) top v o =3.375v lower v o =1.25v 30% 40% 50% 60% 70% 80% 90% 100% 1 10 100 1000 10000 ma figure 1 ? LX1911 circuit topology and typical efficiency performance package order info ld plastic mo-229 6 pin 3mm2 t a ( c) input voltage output voltage range rohs compliant / pb-free 0 to 70 4.5v ? 5.5v adjustable LX1911cld part markings for tape & reel, append the letters ?tr? to the part number. (i.e. LX1911cld-tr) l l x x 1 1 9 9 1 1 1 1
microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 2 copyright ? 2004 rev. 1.1, 2006-05-15 www. microsemi . com LX1911 1.5a, 1.1mh z pwm step-down regulator p roduction d ata s heet tm ? absolute maximum ratings input volta ge (in) ............................................................................................................. .-0.3v to 7.0v sw to gnd.............................................................................................................-0.3v to (v in + 0.3v) v fb to gnd........................................................................................................................ ..-0.3v to +2v sw peak current (int ernally li mited) ....................................................................................... 1800 ma operating temper ature range.......................................................................................-40 c to +125 c storage temperature range, t a ...................................................................................... -65 c to 150 c maximum junction temperature................................................................................................... .150 c rohs / pb-free package peak solder reflow temperature (40 seconds maxi mum expos ure) .......................................................................................260 c (+0,-5) note: exceeding these ratings could cause damage to the device. all voltages are with respect to ground. currents are positive into, ne gative out of specified terminal. thermal data ld plastic micro lead 6-pin thermal resistance - junction to a mbient , ja 25-40 c/w ? junction temperature calculation: t j = t a + (p d x ja ). the ja numbers are guidelines for the thermal performan ce of the device/pc-board system. all of the above assume no ambient airflow. ? pcb layout dependent package pin out fb sw n.c. 1 2 3 4 5 6 gnd vin analog vin pwr ld p ackage (top view) note: heatsink pad on bottom should be connected to ground or left floating. n.c. ? no connect rohs / pb-free 100% matte tin lead finish functional pin description n ame d escription vin analog unregulated supply voltage input, ranging from +4v to 6.0v for internal analog control circuitry. vin pwr unregulated supply voltage input (+4v to 6.0v), high current path, connects to pmos source of pwm switch. fb feedback input for setting programming output voltage. gnd circuit ground providing bias for ic operation and high frequency gate drive bias, can be connected to heatsink terminal. sw inductor and commutation diode connection point. connects to internal pmosfet source. electrical characteristics specifications apply over j unction temperature of: 0 o c < t j < 125 o c for v in = 5v (except where otherwise noted). typical values are at t a =23 c. parameter symbol test conditions min typ max units ` operating range v in functional operation guaranteed by design 4.5 5.5 v feed back threshold v fbt 4.0v < v in < 6.0v 784 800 816 mv fb input current i fb v fb = 0.81v 40 75 na error amplifier bw closed loop 100 khz quiescent operating current i q (pin 5) v fb > 0.825v, rload switch pin < 1k ohms 500 850 a soft start, vout slew rate vo initial power on or after short circuit 21 50 v/ms p-channel switch on resistance r ds(on) i sw = 1.0a 0.25 0.5 maximum duty cycle d i sw = 1.0a (assured by design, not ate tested) 80 100 % sw leakage current i leak v fb = 0.825v 0.01 5 a p-channel current limit i lim peak current at switch pin (not dc current) 1.6 1.9 a pwm frequency f op-pwm pwm mode 700 1020 1400 khz pfm mode region io pfm mode 250 ma feed back psrr 1hz < frequency vin < 10khz -40 db closed loop load regulation load reg v o = 1.2v, 50ma < i o < 1.2a, ckt figure 1 0.85 %v o thermal shutdown t sd (assured by design, not ate tested) 135 150 c e e l l e e c c t t r r i i c c a a l l s s
microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 3 copyright ? 2004 rev. 1.1, 2006-05-15 www. microsemi . com LX1911 1.5a, 1.1mh z pwm step-down regulator p roduction d ata s heet tm ? simplified block diagram vin cl clock and ramp pwm ea gnd bg ref 800mv fb s r sw figure 2 ? LX1911 block diagram inductor diode c out c out c out c out c in vin pwr vin analog n.c. fb gnd sw LX1911 rf rf c s r s ground v in v out figure 3 ? pcb layout considerations a a p p p p l l i i c c a a t t i i o o n n s s
microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 4 copyright ? 2004 rev. 1.1, 2006-05-15 www. microsemi . com LX1911 1.5a, 1.1mh z pwm step-down regulator p roduction d ata s heet tm ? application note f unctional d escription the LX1911 is a current mode pwm regulator with internal compensation. the internal pmos high side switch is protected with current limit on a pulse by pulse basis and with thermal shutdown. thermal shutdown is activated with a junction temperature of 160c (typical) and has 20c of hysteresis. the regulator has an internal power on reset delay of 50-100s to ensure all circuitry is operating before enabling the switch output. soft start is activated upon initial power-on, or following recovery from either thermal shutdown or short circuit. the soft start control block generates a voltage ramp that clamps the error amplifier non-inverting reference voltage. as this clamp voltage rises, the duty cycle is gradually increased, thus limiting the peak inrush currents. pwm / pfm mode of operation is determined by the load current condition. the pfm mode increases system efficiency by reducing the switching frequency thus switching losses. during light loading, i out < 200ma typically, pfm mode becomes active, the switching frequency begins to decrease, the frequency change occurs over a continuous range, decreasing further as i out decreases. o utput v oltage p rogramming resistors r1 and r2 program the output voltage. the value of r2 (lower resistor of divider) should be less than 10k . the value of r1 can be determined using the following equation, note v ref is also referred to as v fbt . 1 v v r2 r1 ref out ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? = d iode s election a schottky diode is required for switching speed and low forward voltage. efficiency is determined mostly by the diode?s forward voltage. the diode conducts 1-d%, for v out = 1.2v this becomes 76% in a 5v system. i nductor s election selecting the appropriate inductor type and value ensures optimal performance of the converter circuit for the intended application. a primary consideration requires the selection of an inductor that will not saturate at the peak current level. emi, output voltage ripple, and overall circuit efficiency affect inductor choice. the inductor that works best depends upon the application?s requirements and some experimentation with actual devices in-circuit is typically necessary to make the most effective choice. i nductor s election , cont . the internal compensation of the LX1911 design was optimized for a 3.3h inductor but will operate with an inductor value in the range of 2.2h and 22h for a majority of applications. the benefit of a larger inductor value can increase efficiency at the lowe r output currents and reduces output voltage ripple, thus output capacitance related to ripple filtering. smaller inductors typically provide smaller package size (critical in many portable applications) at the expense of increasing output ripple current. regardless of inductor value, selecting a device manufactured with a ferrite-core produces lower losses at higher switching frequencies and thus better overall performance. larger inductors may lead to diminished step-load response. c apacitor s election to minimize ripple voltage, output capacitors with a low series resistance (esr) are recommended. multi-layer ceramic capacitors with x5r or x7r dielectric make an effective choice because they feature small size, very low esr, a temperature stable dielect ric, and can be connected in parallel to increase capacitance. typical output capacitance values of 10 to 30f has proven effective. other low esr capacitors such as solid tantalum, specialty polymer, or organic semiconductor, make effective choices provided that the capacitor is properly rated for the output voltage and ripple current. finally, choose an input capacitor of sufficient size to effectively decouple the input voltage source impedance (e.g., c in > 4.7f). l ayout c onsiderations the high peak currents and switching frequencies present in dc/dc converter applications require careful attention to device layout for optimal performance. basic design rules include: (1) maintaining wide traces for power components (e.g., width > 50mils); (2) place c in , c out , the schottky diode, and the inductor close to the LX1911; (3) minimizing trace capacitance by reducing the etch area connecting the sw pin to the inductor; and (4) minimizing the etch length to the fb pin to reduce noise coupling into this high impedance sense input. other considera tions include placing a 0.1uf capacitor between the LX1911 v out pin and gnd pin to reduce high frequency noise and decoupling the vin pin using a 0.1f capacitor. the LX1911 switch has fast switching speeds which may generate noise spikes when a high capacitance schottky diode is selected for the catch diode. a simple snubber circuit, as shown in figure 1, r=10 ohms and c=680pf has proven effective to reduce the spike voltage generated at the switch pin / diode connection. a a p p p p l l i i c c a a t t i i o o n n s s
microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 5 copyright ? 2004 rev. 1.1, 2006-05-15 www. microsemi . com LX1911 1.5a, 1.1mh z pwm step-down regulator p roduction d ata s heet tm ? characteristic curves figure 4? step load response 250ma-1.2a 3.3h, 40f blue: vout 50mv/div ac; green: i step 200ma/div figure 5 ? switching waveforms: pfm mode figure 6 ? power on and soft start for i out 10ma to 1amp regulated vout vs temperature (iout=10ma) -0.8% -0.6% -0.4% -0.2% 0.0% 0.2% 0.4% 0.6% 0.8% -25 0 25 50 75 100 125 150 temperature, c figure 7? vout temperature stability ch1:vin ch2:vout ch2:vout ch1:switch g g r r a a p p h h s s
microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 6 copyright ? 2004 rev. 1.1, 2006-05-15 www. microsemi . com LX1911 1.5a, 1.1mh z pwm step-down regulator p roduction d ata s heet tm ? package dimensions ld 6 pin plastic 3 x 3 x .9 mm d e a1 a l1 l e d2 e2 b k pin 1 id pin 1 id or or note: 1. dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm(.006?) on any side. lead dimension shall not include solder coverage. m illimeters i nches dim min max min max a 0.80 1.00 0.031 0.039 a1 0.00 0.05 0.000 0.002 k 0.20 min 0.008 min e 0.95 bsc 0.037 bsc l 0.30 0.50 0.012 0.02 b 0.30 0.45 0.012 0.018 d2 1.90 2.40 0.75 0.094 e2 1.15 1.65 0.045 0.065 d 3.00 bsc 0.118 bsc e 3.00 bsc 0.118 bsc l1 0.00 0.15 0.000 0.006 ld 6 pin plastic 3 x 3 x .9 mm d e a1 a l1 l e d2 e2 b k pin 1 id pin 1 id or or note: 1. dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm(.006?) on any side. lead dimension shall not include solder coverage. m illimeters i nches dim min max min max a 0.80 1.00 0.031 0.039 a1 0.00 0.05 0.000 0.002 k 0.20 min 0.008 min e 0.95 bsc 0.037 bsc l 0.30 0.50 0.012 0.02 b 0.30 0.45 0.012 0.018 d2 1.90 2.40 0.75 0.094 e2 1.15 1.65 0.045 0.065 d 3.00 bsc 0.118 bsc e 3.00 bsc 0.118 bsc l1 0.00 0.15 0.000 0.006 m m e e c c h h a a n n i i c c a a l l s s
microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 7 copyright ? 2004 rev. 1.1, 2006-05-15 www. microsemi . com LX1911 1.5a, 1.1mh z pwm step-down regulator p roduction d ata s heet tm ? notes production data ? information contained in this document is proprietary to microsemi and is current as of publication date. this document may not be modified in any way without the express written consent of microsemi. product processing does not necessarily include testing of all parameters. microsemi reserves the right to change the configuration and performance of the product and to discontinue product at any time. n n o o t t e e s s
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